Permanent magnet materials are often used in devices to transform electrical energy into mechanical. In these devices, mechanical force is generated by the interaction of the magnetic field of a permanent magnet and the magnetic field generated by electrical current flowing through a coil. A typical example of such a device is a permanent magnet motor where current flowing through a winding generates a field which interacts with a field generated by a permanent magnet to drive a rotor. Another example is a loudspeaker where a winding is freely movable with respect to a permanent magnet. An amplified audio signal passes through the winding and the resulting magnetic field causes the winding and a loudspeaker cone to generate an audio signal. The permanent magnet is thus often used with a counteracting external field.
For the stability of the device, it is desirable to know that the permanent magnet material used within the device can withstand the external magnetic field without deterioration of its magnetic state. With increasing strength of the counteracting magnetic field, the magnetisation direction becomes unstable and the magnetisation will ultimately reverse. The ability to withstand an external field is described in the technical literature with a set of curves of the magnetic moment per unit volume (the magnetisation M) as a function of the magnetic field H. These curves are called hysteresis curves as they describe the memory of the magnetic state of a material and its resilience to change.
For typical applications, it is the behavior of the magnetisation combined with a counteracting magnetic field which is of importance. On the graph of magnetisation versus external field, this region of importance is located in the second quadrant. The parameter describing the magnetic field required to start reversing the magnetisation of the material is the coercive field HcJ. FIG. 1 shows the second quadrant of a typical hysteresis curve for a permanent magnetic material.
The advent of strong permanent magnetic materials such as NdFeB and SmCo created a revolution in devices because magnetic fields could be generated with significantly less material, thus saving weight and volume. Using such materials, it is now possible to produce a permanent magnet material in which the coercive field is greater than 2 MA/m.
In order to produce a hysteresis graph it is necessary to test a magnetic material by generating a magnetic field (i.e. a coercive field) over a wide range of strengths. With coercive fields above 2 MA/m, the generation of the magnetic field capable of measuring the full hysteresis curve requires special techniques. Conventional laboratory electromagnets with table-top sized iron yokes and pole shoes can achieve a field of 1.6 MA/m. Above this figure, generating the field can require in excess of 10 kW of electrical power. Superconducting magnets can achieve fields in the range 10 MA/m but they require special cryogenics. They are therefore relatively expensive in purchase and running costs. Magnetic fields generated from pulsed power supplies such as capacitor banks circumvent the need for large installed electrical power as the power is derived from an energy storage power supply. The consequence is that the pulse is limited in time by the available stored energy. Present day pulsed field installations at research facilities reach fields up to 50 MA/m and above. Laboratory equipment based on compact capacitive discharges reach fields up to 25 MA/m. In view of this, it is desirable to generate the magnetic fields required for measuring the complete hysteresis of highly coercive materials by using capacitive energy storage discharges.
One known way of measuring the magnetic moment of a sample material is to measure the magnetic flux emanating from the sample using pick-up coils. The voltage induced in the pickup coils (e.g. integrated over time) is proportional to the magnetic moment. One of the most common pick-up coil systems is the Helmholtz coil pair. The Helmholtz coil pair geometry is a special geometry for which the response is rather insensitive to the sample position, i.e. the displacement from the center position is only visible in the fourth order. Typically, the displacement can be as large as half the radius of the coils before the response changes by 1%. Higher order compensation is possible using more coils. The insensitivity to sample position of the Helmholtz coil is used both for measuring large sample sizes and for coarse sample positioning.
The Helmholtz coil is a pair of identical circular coils with a common axis. The distance of the coils is equal to the radius of the coils. Coil sets with a smaller distance have sensitivity similar to a single coil, with a maximum at the center and decreasing when moving away on the axis. The response as a function of the axial distance shows a negative second order dependence. For coil sets with a larger distance, the response is similar to the response of two independent coils, showing a maximum near the center of each coil and a minimum at the middle. Due to the increase of the sensitivity when moving away from the middle and in the direction of each coil center, the sensitivity shows positive second order dependence. FIG. 2 schematically shows the geometry of Helmholtz coils, taking a section through the coils in the half-plane. The axis of rotation is located at the left; the coils are shown as two squares. The intensity of the response is shown as a grayscale ranging from 95% to 105% with steps every 0.5%.
Alternative methods for measuring the magnetic moment are measuring the force on the sample when placed in a gradient of a magnetic field. Another method is to measure the magnetic flux density (B field) generated by the sample at a fixed position. This method is geometry dependent.
The method of measuring the magnetic moment using a Helmholtz coil pair can only be used when there is a steady background field. Due to its geometry, the Helmholtz coil pair is an even better sensor for the flux of a background field than for the flux of a magnetic sample in the center of the coils. Variations of the background field therefore generate a parasitic signal in the Helmholtz coil pair, or in any coil. However, as explained above, the high coercive field needed to generate a full hysteresis curve requires techniques for which the background field is not constant.
For measurement of the magnetic moment in a changing background field, compensation techniques are required for any method based on the inductive measurement. Compensation would be based on the use of two coil sets with different response: one coil set to measure the more of response of the sample and the other coil set to measure more of the response of the background field. The proper signal of the sample needs to be reconstructed by subtracting the latter from the first. However, if the first set would have Helmholtz geometry, the signal of the second set would be sensitive to the position of the sample. The difference of both signals would show increased position sensitivity.
U.S. Pat. No. 5,134,370 describes an apparatus for detecting the presence of a magnetic tag in a specimen. The apparatus comprises a plurality of magnetic field detecting coils which have outputs balanced against one another in a uniform magnetic field and in some instances also balanced in a field with uniform gradient. The coils are separated from one another so that at least one coil is disposed closer to the specimen under test than at least a second coil. The apparatus measures gradient of nearby source and the compensation technique is to balance in uniform background field. Measurement of gradient makes the sensor sensitive to the position of the sample under test. The coils are moved past the stationary specimen under test. If a tag is present in the specimen in the area under examination, the multiple coils will detect different magnetic fields. The output of the balanced coils system will no longer indicate cancelled fields. Instead, the coils will indicate the detection of a difference between the magnetic fields which effect the different coils. Once the difference is detected, a signal is generated to advise the user of the existence of the additional magnetic field and thus the existence of the magnetic tag in the specimen in question.
GB 2 382 149 provides a method for measurement of desired magnetic gradients from a far target source based upon direct magnetic gradiometers, where an immunity to near noise-like magnetic signals is provided. The gradiometer comprises two first direct magnetic gradiometers placed side by side and having their sensitivity axes aligned along a same x-direction in order to distil near signals from far signals and a complementary pair of direct magnetic gradiometers placed along a y-direction. Each of the four direct magnetic gradiometers is placed vertically and measures one of the off-diagonal components of the magnetic gradient tensor. By differencing the output of the two first direct magnetic gradiometers an output signal containing information on the near signals but not on the far signals is obtained. The device according to GB 2 382 149 measures a gradient of a nearby source and the compensation technique is balanced using a (uniform) reference signal.
US 2004/0196035 describes a magnetometer in which structure dissymmetries, such as irregularities of the diameter of the wire or the turns, or even the number of turns, between the windings in a receiver coil or their relative positions with respect to excitation coils that have an influence on the mutual inductance between the excitation and induction coils, are corrected. The magnetometer according to US 2004/0196035 measures a gradient of a nearby source and the compensation is performed electronically.
Up to now, no sensor was developed which shows both proper background compensation and the position insensitivity of the Helmholtz coil pair.
The present invention seeks to improve the accuracy of measuring the magnetic flux or moment of a sample, particularly in a varying background field.